台灣南部海域星狀軟珊瑚之有性生殖及族群動態研究

Abstract

星狀軟珊瑚（Asterospicularia laurae）屬於傘軟珊瑚科星狀軟珊瑚屬，其模式種的產地在南灣，但是目前僅分布於恆春半島西岸的合界淺海。除台灣之外，也僅在澳洲大堡礁有分佈，有關其生活史特徵及族群動態資料皆闕如。本研究針對合界的星狀軟珊瑚族群，自2007年12月至2009年3月，以定期採樣及組織切片觀察，探究星狀軟珊瑚有性生殖特徵，並且利用追蹤固定樣區內珊瑚群體變動的方式，探討其族群動態。有性生殖研究結果顯示，星狀軟珊瑚屬於雌雄異體，體內孵育型的生殖模式，成熟卵卵徑約為260-300 μm，最小性成熟體型介於4-8 cm2，比起其他科軟珊瑚屬於較早性成熟的軟珊瑚種類。雌雄性比約為1:1，自1月底至7月初皆可發現珊瑚蟲體內有配子發育，其他月份則無，因此其生殖季約在每年的2至7月；並且自2008年4月中至6月皆可在共肉組織內觀察到幼生，此期間可能皆有幼生釋放。族群動態的結果顯示，星狀軟珊瑚族群大部分時間以小群體（＜4 cm2）為主，當環境穩定時，由於群體的成長速率高及群體間的融合作用，因此族群結構可在短時間內轉變為大群體（＞64 cm2）為主的組成；而群體的總覆蓋面積也大幅增加。有性生殖加入量為0.07-0.36 colony/m2•mo，僅占族群數量1.52-8.20%，顯示有性生殖入添對於族群的貢獻不高。底棲群聚分析結果顯示，該區底質大部分由藻類覆蓋（＞50%），藻類可能與星狀軟珊瑚競爭空間。當颱風季節過後(2008年9月)，星狀軟珊瑚的體型頻度分布及總覆蓋面積都呈現明顯變動，體型頻度集中在中間體型（4-64 cm2），總覆蓋面積大幅減少（減少約28%），而底棲藻類的比例降低，空白底質則增加。然而，2009年3月的調查時，星狀軟珊瑚族群的覆蓋率高達6.21%，顯示其在干擾後的復原快速。研究結果顯示，星狀軟珊瑚族群易受干擾的影響而有所大幅波動，其族群的維持大部分依靠群體成長及無性分裂生殖。由於有性生殖入添的成功率低，而無性分裂生殖產生的小群體散佈能力有限，因此台灣的星狀軟珊瑚族群呈現出分布侷限於合界的現象。

Asterospicularia laurae (Utinomi 1951), a species of Xeniidae, has a restricted distribution on the shallow reef fronts at Hejie on the west coast of Hengchun Peninsula, southern Taiwan. Besides Taiwan, A. laurae has only been found in the Great Barrier Reef, Australia. This disjunct distribution is of particular interest. However, the information on life-history traits and ecology of A. laurae are lacking. In this thesis, the sexual reproduction and population dynamics of A. laurae at Hejie were studied by collecting samples every 1.5-2 months for histological studies and monitoring the changes of coral colonies in 5 belt transects from December 2007 to March 2009. Asterospicularia laurae is a gonochoric internal brooder, and the sex ratio is approximately 1:1. The smallest colony size of sexual maturation is 4-8 cm2, which indicates an early maturated case in octocorals. Gammetogensis was observed from late January to early July suggesting that the reproductive season is likely from February to July. Mature oocytes were approximately 260-300 μm in diameter. The planulae were found in coenenchyme from mid April to June thus planulation may occur during this period. The size-frequency distribution of A. laurae showed that the population was mainly composed of small colonies (< 4 cm2) during most of the sampling period, and then shifted to the dominance of large-sized colonies (> 64 cm2) in a short time due to the high intrinsic growth rate of colonies and fusion events. As a result, the total coverage also increased dramatically. The number of sexual recruits was 0.07-0.36 colonies/m2 per month and comprised approximately 1.5% to 8.2% of the total number of colonies throughout the study period. It revealed the contribution of sexual recruits to the population may be low. Benthic community associated with A. laurae was mainly composed of macroalgae (>50%). After the typhoon season, the size-frequency distribution were more centralized in the medium-sized colonies (4-64 cm2) due to the high whole-colony mortality of small colonies and high partial mortality of large colonies and the total coverage reduced by 28%. Meanwhile, the coverage of macroalgae decreased and the bare substrate increased. However, in March 2009, the total coverage of A. laurae was 6.21%, the highest during the study period, indicating the rapid recovery of A. laurae after disturbance. This study concluded that the population of Asterospicularia laurae in the area is highly contributed by asexual reproduction, reflecting an adaptation in a wave-swept and frequently disturbed habitat. However, due to the limited dispersal ability of asexual recruits and possibly high pre- and post-settlement mortality of sexual recruits, distribution of A. laurae was restricted distribution at Hejie, southern Taiwan.